1. Field of the Invention
The present invention is generally directed to a method for forming combinatorial libraries comprising arrays of materials prepared by depositing a dissolved metal on a support for use as catalysts, such as electrocatalysts. The invention is also directed to combinatorial libraries comprising an array of such metal-containing supported catalysts. These catalyst-containing libraries are particularly well-suited for use in conducting combinatorial research investigations, in particular with respect to electrocatalysts for fuel cells.
2. Background Information
A fuel cell is an electrochemical device for directly converting the chemical energy generated from an oxidation-reduction reaction of a fuel such as hydrogen or hydrocarbon-based fuels and an oxidizer such as oxygen gas (e.g., in air) supplied thereto into a low-voltage direct current. For the oxidation and reduction reactions in a fuel cell to proceed at useful rates, especially at operating temperatures below about 300° C., electrocatalyst materials are typically supplied at the electrodes. Initially, fuel cells used electrocatalysts made of a single metal, usually platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os), silver (Ag) or gold (Au) because they are able to withstand the corrosive environment, platinum being the most efficient and stable single-metal electrocatalyst for fuel cells operating below about 300° C. Although platinum is the most efficient and stable single-metal electrocatalyst for fuel cells, it is costly and an increase in electrocatalyst activity over platinum is generally considered to be necessary for wide scale commercialization of fuel cell technology. An improvement in catalysts may take many forms such as increased activity, increase corrosion resistance, increased poison tolerance, and/or decreased costs. For example, increased tolerance to CO has been reported by alloying platinum and ruthenium at a 50:50 atomic ratio (see, D. Chu and S. Gillman, J. Electrochem. Soc. 1996, 143, 1685). Ideally, a reduction in cost will accompany an improvement in one or more of the preceding performance characteristics.
Fuel cell electrocatalysts were first used in fuel cells in metallic powder form. However, techniques have been developed to disperse these metals over the surface of electrically conductive supports (e.g., carbon black) to increase the surface area of the electrocatalyst which in turn increased the number of reactive sites leading to improved efficiency of the cell.
Combinatorial materials science is known in the art, and generally comprises library design, library synthesis, optionally characterization of one or more members of the library, and library screening. In particular, combinatorial materials science, or more specifically combinatorial chemistry, refers generally to methods for synthesizing a collection of chemically diverse materials and to methods for rapidly testing or screening this collection of materials for desirable performance characteristics and properties.
Combinatorial chemistry approaches have greatly improved the efficiency of discovery of useful materials. For example, material scientists have developed and applied combinatorial chemistry approaches to discover a variety of novel materials, including for example, high temperature superconductors, magnetoresistors, phosphors and catalysts. (See, for example, U.S. Pat. No. 5,776,359 to Schultz et al.) In comparison to traditional materials science research, combinatorial materials research can effectively evaluate much larger numbers of diverse compounds in a much shorter period of time.
Although such high-throughput synthesis and screening methodologies are conceptually promising, substantial technical challenges exist for application thereof to specific research and commercial goals. For example, the known approaches for preparing combinatorial libraries have not been particularly adapted for preparing supported catalysts, especially supported electrocatalysts having relatively high metal loadings, and especially both relatively high metal loadings and relatively narrow particle size distribution.